Utilization of an autopolymerizing organosiloxane-based compound

ABSTRACT

The invention relates to a liquid or pasty organosiloxane based composition autopolymerizing into a permanently elastic silicon at ambient temperature. This composition is used for producing intervertebral disc implants that restore to a large extent the physiological range of movement of the intervertebral discs.

By his or her thirtieth year, the intervertebral discs in every humanbeing exhibit degenerative changes. Disc prolapses necessitate surgicalintervention; however, forays into the lumbar spine region can sometimessuffer from a high failure rate. The amount of recurrent intervertebraldisc prolapse and Failed Back Surgery Syndrome or postnucleotomysyndrome is relatively high. After a nucleotomy, often following aproblem-free interval. Increasing degeneration of the segment that hasbeen operated upon and the degeneration of the bending joints are seenas the main source of long term deterioration. This means that one ofthe most important causes for complaint after intervertebral discoperations is the increased mobility of the vertebral motor segment. Apossible solution is to stiffen the vertebral motor segment with therisk of increasing the load on the bordering segments and the danger ofinstability of the connectors. A further possibility is to implant anartificial intervertebral disc replacement, for example of a hydrophilicmaterial packaged in a polyethylene net that is implanted in the drycondition.

Tests using a model of the spinal column have shown that a major causeof such long-term deterioration after intervertebral disc operations isthe increased mobility of the vertebral motor segment. Thus the aim ofthe invention is to provide intervertebral disc implants that reproducethe physiological amount of movement of the disc as far as possible. Theinvention thus concerns the use of a fluid or pasty organosiloxane-basedcomposition which is self-polymerisable at ambient temperature to apermanently elastic silicone for the production of intervertebral discimplants.

The disc consists of an outer zone, the anulus fibrosus, with theso-called Sharpey fibres and a gel-like parachondral inner zone, thenucleus pulposus. In accordance with the invention, theself-polymerisable organosiloxane-based composition is solely used forthe production of nucleus pulposus implants, while the anulus fibrosusremains unaltered. Since the composition as used in the invention isfluid or pasty, after removing the gel-like inner zone from the anulusfibrosus, this composition can be introduced into the space thus formedand then undergoes self-polymerisation. On the one hand, then the anulusfibrosus acts to maintain the spacing of the vertebral segments, and onthe other hand it simultaneously acts as a mould for the fluid or pastycomposition of the invention, which is injected into this mould. Aminimal endoscopic invasive entry to the intervertebral disc via theretroperitoneal space is sufficient from the ventral or, as is standard,dorsal direction. After removing the nucleus pulposus and introducingthe hydrophobic organosiloxane-based composition of the invention, theanulus fibrosus can then be closed. The composition polymerises withoutadditional heating to a permanently elastic silicone and completelyfills the hollow space within the anulus fibrosus, providing a maximumsurface area for load transfer, so that an even movement pattern isproduced. Thus the invention means that the segmental stability of theso nucleotometrised intervertebral segment can be reinstated.

The organosiloxane-based composition as used in the invention must besterile for use. Since such sterilisation usually cannot be carried outby the user of the fluid or pasty composition, the composition must besterilised by the manufacturer and sold as internally sterile packages.

The biocompatibility of the compositions as used in the invention mustbe as high as possible in addition to having the highest possiblecontinuous load capacity. It has been shown that on self-polymerisation,polydimethylsiloxane-based compositions produce silicones which possessboth properties. Since the intervertebral disc is subjected toconsiderable pressure loads, this result was surprising and notpredictable. The permanently elastic silicones produced frompolydimethylsiloxanes have no deleterious effect on the surroundingtissue, on cell proliferation and on normal cell division. Degenerativechanges were not observed. Linear polydimethylsiloxane-basedcompositions are particularly advantageous.

By copolymerising the methylsiloxane with differently substitutedsiloxanes, the reaction period for self-polymerisation and theproperties of the silicone produced can be varied. As an example,instead of methyl groups, phenyl residues, ethyl groups or vinylresidues can be included, to produce mixed methylphenyl-, ethylmethyl-or methylvinyl-siloxanes, for example. Such co-polymerisates can havetheir substituents randomly arranged or arranged as a block copolymer.The end groups of the polydimethylsiloxane are preferablytrimethylsiloxy groups, but at least a portion thereof can be silanolgroups, vinyl groups or hydride groups. The viscosity of the linearpolydimethylsiloxane can be governed by chain breaking groups, essentialfor the invention as for use, the compositions must be fluid or pasty.

In order to be self-polymerisable at ambient temperature, thecompositions of the invention comprise a catalyst. Known siloxanecatalysts are amines, such as aminopropylsilane derivatives, and lead,tin and zinc carbonic acid salts, also organic iron, cadmium, barium,antimony or zirconium salts. Tin octoate, laurate and oleate as well asdibutyltin salts are particularly suitable. The selected catalysts mustbe biologically compatible. These include addition catalysing noblemetal complexes from group VIII, such as platinum, rhodium or ruthenium,which catalyse self-polymerisation within suitable polymerisationperiods in very small amounts, for example concentrations as low as 1 to2 ppm. In particular, platinum catalysts such as platinum-olefincomplexes can catalyse addition of Si—H end groups to olefins such asvinyl functional siloxanes. Preferred compositions which polymerise topermanently elastic silicones are produced from mixtures of linearhydride functional polydimethylsiloxanes and linear vinyl functionalpolydimethylsiloxanes mixed with a suitable catalyst, in particular aplatinum catalyst such as chloroplatinic acid or another platinumcompound. In order to avoid premature polyaddition, such compositionsare stored in two separate packs and are only combined immediately priorto use. Normally, the catalyst is stored together with the vinylfunctional polydimethylsiloxane in one package and the hydridefunctional polydimethylsiloxane, optionally with the usual hardeners, ifnecessary with the addition of a vinyl functional polydimethylsiloxanebut without the catalyst, is stored in the second package. The ratios ofthe amounts of both polydimethylsiloxanes used depends on the desiredproperties of the permanently elastic silicone.

Moisture from the air could also be used as the catalyst, however thisis not important because the composition must be sterile for use inaccordance with the invention.

The average molecular weight (number average) of the compositions of theinvention is in the range 1000 to 150000, preferably in the range 50000to 100000.

The compositions of the invention can also comprise the usual fillerssuch as finely divided silica such as fly ash, whereby the maximumparticle size of such fillers should be 1 μm, for example in the range0.01 to 0.5 μm. Other fillers which can be considered are dehydratedsilica gels, diatomaceous earth, finely divided titanium dioxide,aluminium oxide or zirconium oxide. Finally, the usual process materialscan be worked into the organosiloxane masses of the compositions of theinvention, provided that they are biocompatible and do not affect thedesired properties of the polymerised silicone, in particular itspermanent elasticity and compression resistance.

What is claimed is:
 1. A method for producing an intervertebral discimplant for improving the physiological amount of movement in anintervertebral disc damaged by degenerative changes, the methodcomprising: removing the gel-like inner zone from the annulus fibrosusof the intervertebral disc to form a space, introducing a hydrophobic,noble metal catalyst selected from Group VIII containingself-polymerizable fluid or pasty organosiloxane-based composition intothe space; and permitting at ambient temperature the composition toself-polymerize to a permanently elastic silicone which completely fillsthe hollow space within the annulus fibrosus.
 2. A method according toclaim 1, wherein the intervertebral disc implant is a nucleus pulposusimplant.
 3. A method according to claim 1, wherein the composition isbased on dimethylsiloxane.
 4. A method according to claim 1, wherein thecomposition includes a platinum catalyst.
 5. A method according to claim2, wherein the composition includes a platinum catalyst.
 6. A methodaccording to claim 3, wherein the composition includes a platinumcatalyst.
 7. A method according to claim 4, wherein the composition isbased on hydride-functional siloxanes and vinyl-functional siloxanes. 8.A method according to claim 5, wherein the composition is based onhydride-functional siloxanes and vinyl-functional siloxanes.
 9. A methodaccording to claim 6, wherein the composition is based onhydride-functional siloxanes and vinyl functional siloxanes.
 10. Amethod according to claim 2, further comprising sterilizing thecomposition.
 11. A method according to claim 3, further comprisingsterilizing the composition.
 12. A method according to claim 4, furthercomprising sterilizing the composition.
 13. A method according to claim5, further comprising sterilizing the composition.
 14. A methodaccording to claim 6, further comprising sterilizing the composition.15. A method according to claim 7, further comprising sterilizing thecomposition.
 16. A method according to claim 8, further comprisingsterilizing the composition.
 17. A method according to claim 9, furthercomprising sterilizing the composition.
 18. A method according to claim1, wherein the composition is sterile.